Field Of the Invention
The present invention relates to the pumping of laser systems based on crystals with the chemical formula, Ce-doped XYZF.sub.6, where X, Y and Z are respectively singly, doubly, and triply-charged cations. The invention relates particularly to the use of pump light near 266 nm that is polarized along the c axis of a Ce-doped LiSrAlF.sub.6 crystal.
While numerous useful solid state lasers have been developed that provide infrared output, the identification of effective visible and ultraviolet-capable solid state laser gain media has proved to be far more problematic. For example, the well-known Nd:YAG laser operates at 1064 nm in the near infrared. The wavelength of operation is directly determined by the nature of the energy levels of the laser ion in the crystal--in this case it is the Nd.sup.3+ ion in the Y.sub.3 Al.sub.5 O.sub.12 host (known as YAG). Other common rare-earth laser ions include Er.sup.3+, Tm.sup.3+ and Ho.sup.3+ ; the important laser wavelengths characterizing these ions are once again the infrared, in the 2000-3000 nm region. Lasers based on transition metal ions rather than rare earth ions also tend to operate in the infrared region. For example, Ti.sup.3+ -doped sapphire (Al.sub.2 O.sub.3) has proved to be a very useful laser material that is capable of generating tunable near infrared radiation in the range of 750-1100 nm [P. F. Moulton, "Spectroscopic and laser characteristics of Ti:Al.sub.2 O.sub.3," Journal of the Optical Society of America B 3, 125-133 (1986)]. There have been numerous examples of solid state laser materials that generate laser light in the visible and ultraviolet regions, but none of these lasers has thus far been judged to be adequately robust, efficient and useful so as to become commonly employed within industry or the scientific community.
The unavailability of ultraviolet-generating gain media suddenly changed when Ce-doped LiCaAlF.sub.6 was reported to lase reliably [M. A. Dubinskii,et al. "Ce.sup.3+ -doped colquiriite. A new concept of all-solid-state tunable ultraviolet laser," J. Modern Optics 40, (1) 1-5 (1993); M. A. Dubinskii, et al., "Spectroscopy of a New Active Medium of a Solid State UV Laser with Broadband Single-Pass Gain," Laser Physics 3, (1) 216-217 (1993)]. Here, Ce:LiCaAlF.sub.6 was pumped with the fourth harmonic 266 nm output of a Nd:YAG laser and demonstrated to yield gain and to provide laser oscillation when appropriately arranged in a cavity. However, the effect of polarization of the pump source was not considered, and no preferred polarization was indicated.
The significance of the work by Dubinskii, et al., can be understood by reviewing the historical context of solid state lasers based on the Ce.sup.3+ ion. Initial attempts to identify a viable Ce.sup.3+ laser involved the Y.sub.3 Al.sub.5 O.sub.12 host [Jacobs, et al., "Measurement of excited-state-absorption loss for Ce.sup.3+ in Y.sub.3 Al.sub.5 O.sub.12 and implications for tunable 5d-4f rare-earth lasers," Applied Physics Letters 33, (5) 410-412 (1978); Miniscalco, et al., "Measurement of excited-state absorption in Ce.sup.3+ :YAG," J. Physics 49, (12) 6109-6111 (1978)]. These original efforts ended in failure in that there was no gain found in Ce:YAG as a consequence of excited state absorption at the emission wavelengths. In other words, for the case of Ce:YAG the pump laser induces loss at the emission wavelengths rather than gain.
Ce.sup.3+ was later found to exhibit gain in several fluoride hosts, including LiYF.sub.4, LaF.sub.3 and related crystals (Ehrlich, et al., "Ultraviolet solid-state Ce:YLF laser at 325 nm," Optics Letters 4, (6) 184-186 (1979); Ehrlich, et al., "Optically-pumped Ce:LaF.sub.3 laser at 286 nm," Optics Letters 5, (8) 339-341 (1980)]. While these materials did exhibit laser oscillation, they also proved to be very inefficient and to solarize (i.e., color) easily. The result was that the laser output was weak and could not be operated with an adequate repetition rate. Thus it is desirable to find better Ce.sup.3+ doped laser materials.
Several patents are relevant to the art. The host employed by Dubinskii, et al., LiCaAlF.sub.6, was disclosed in U.S. Pat. No. 4,811,349, although with Cr.sup.3+ dopants. U.S. Pat. No. 4,233,570 discusses the potential use of flashlamps to pump Ce.sup.3+ -laser materials having certain characteristics. U.S. Pat. No. 4,132,962 discloses lasers with Ce-doped fluorides that contain a metal ion from column IIIB of the periodic table. Lastly, U.S. Pat. No. 4,083,018 describes Ce:LiYF.sub.4, Ce:LaF.sub.3, and other related crystals.
It is generally desired to optically pump a laser as efficiently as possible, in anticipation of maximizing the output. This is usually achieved by selecting a wavelength, and in some cases polarization, for which the pump light absorption is maximized. In cases where the absorption is substantially equal for different polarizations, equivalent performance is expected for all polarizations. Therefore, no particular polarization of the pump source is preferred.